The present invention relates to a color developing material, and more particularly to a heat sensitive color developing material which has numerous applications including being used as a recording material.
The heat sensitive color developing material of the present invention is able to develop color at lower temperatures and can be manufactured at low cost when compared to prior art color developing materials.
Conventional heat sensitive color developing materials are usually produced by dispersing an electron donating color forming organic compound and a phenolic compound, as a color developer, into a binder in high concentrations, and then coating it onto paper. As soon as the phenolic compound is fused by heating, a phenolic hydroxyl group allows the electron donating compound to develop color.
However, conventional heat sensitive color developing materials have numerous disadvantages.
Chief among them is the development of color, prior to heating. Generally, when a prior art color developing material is mixed with an electron donating organic compound, it begins to develop color immediately, prior to being heated. This property is based upon the high solubility of the phenolic compound in an organic solvent. This feature makes it impractical to dissolve and mix the color developing materials with each other.
Additionally, it is difficult to uniformly disperse the electron donating organic compound and the phenolic compound into a binder, to wit: in a finely divided form (less than one micron).
Suitable heat sensitivity at a given temperature is maintained by adjusting the melting point of the phenolic compound. Phenolic compounds that can be used to adjust the heat sensitivity are limited in number. Thus suitable heat sensitivity at a given temperature is limited to those phenolic compounds.
In an attempt to overcome the aforementioned problems, Japanese Laid Open Patent Application Publication No. 62-263525 provides for a heat sensitive color developing material using a compound or resin containing a neutral t-butyl ester as the electron donating compound, such as, a leuco compound.
When one of a compound and resin containing the neutral t-butyl ester radical is heated to a temperatures of 200.degree. C. or higher, it decomposes rapidly into carboxylic acid and isobutane. Due to the presence of the carboxylic acid, the leuco begins to develop color. This feature results in an improvement in conventional heat sensitive color developing materials using phenolic compounds as color developing agents.
Japanese Patent Application Publication No. 63-32637 also attempts to overcome the aforementioned disadvantages by proposing a color developer which is resolvable at lower temperatures. This feature was thought to provide a heat sensitive color developing material which is able to develop color at a lower temperature.
The present invention attempts at overcoming the aforementioned problems associated with prior art heat sensitive color developing materials.